Fixes [YOCTO #2808]
Added the first draft of the expanded explanation for generating
an image.
(From yocto-docs rev: 72e4db25d89cad62e1cd9ee1d638af374ec1bfc0)
Signed-off-by: Scott Rifenbark <scott.m.rifenbark@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>